Not too sure about anythinb thourough you should check for on a 2wd but here's a generall guide.

Look around all bodykit fixings for rust. Rear lower arms, the arms themselves and mounting locations. Front and rear bumper mounts. Front and rear aches, usually rusts on the lips.

Check it ain't bent, all gaps are parallel, the bonnet is sitting true. Open up the boot and pull up the carpets, check that the panels are not creased or wrinkled, a sign of a rear end shunt. Also check that the roof linning isn't sagging as this will be an indication of the car having rolled.

Check that none of the wheels are buckled or that there isn't any positive camber (think its positive camber) when the top of the wheel sticks out and the bottom sticks in, this is usually due to power sliding into kerbs, can be very expensive to fix. Lowered sierra's usually have negative camber, top sticks in, bottom sticks out, not to worry about unless its extreme.

Engine wise, you want to see the car cold. On start up listen out for excessive tapping, you'll get some on a high miller, common to pinto's. White smoke (not condensate), blue smoke or black smoke. Overrun when you switch the engine off, signs of turbo death.

Check the rear drive and shafts. The axles shouldn't be loose, there'll be play back and forth but not in a radial direction. Look for leaky oil around the drive and gearbox.

Can't think of much else off the top of my head. Just do the general checks, it is a sierra after all. Head gaskets, loose gear change, clutch wear and so forth.

Hope that helps.